The present invention relates to "heart/lung" machines used during surgical procedures. More particularly, the present invention relates to the recirculation of blood through the heart/lung machine enabling the use of smaller blood oxygenator and/or heat exchange elements.
Delicate surgical procedures require that the site of surgery remain motionless during the surgical process. This requirement made early heart surgery almost impossible, as stoppage of the heart's pumping action for the required length of time would invariably be fatal.
During the 1960's, prolonged non-fatal stoppage of the heart became possible by use of newly developed heart/lung machines. These machines consisted of a mechanical (i.e., first pumps were roller type) blood pump combined with a blood oxygenator. The heart/lung machines were capable of taking over the function of the natural heart and lungs for several hours and thus, enabling the development of techniques leading to today's extensive practice of open heart surgery.
Typically, centrifugal blood pumps derive their pumping action from the rotation of an impeller within a pumping chamber. Pump pressure is controlled by the rotational speed of the impeller. Centrifugal blood pumps are widely recognized as a safe, reliable, mechanical alternative to the roller pump. Although centrifugal blood pumps have a wide range of capacities, all pumps deliver enough blood to sustain a patient during surgery. An oxygenator is used to provide oxygenation of the blood during surgery. The oxygenator is the substitute for the natural lungs. Most currently used oxygenators employ a membrane made of a "microporous fiber bundle". A microporous fiber bundle is a collection of fine, hollow, porous polymer fibers which remove carbon dioxide and transfer oxygen to the blood coming into contact with the fiber bundle. Oxygenation of the blood and removal of carbon dioxide typically occurs when blood is passed over the outside of the fibers, while oxygen passes through the inside of the fibers. The amount of fibers which make up the fiber bundle must be sufficient to accomplish adequate gas transfer during the time that the blood is in contact with the oxygenator. Conventional heart/lung machines also utilize a heat exchanger, as well as a membrane oxygenator and a blood pump, to control the temperature of the patient during and after the surgical process.